Distillate fuels which are intended for use in jet turbines must meet certain minimum standards in order to be suitable for use. Jet fuel must have good oxidation stability in order to prevent the formation of unacceptable amounts of deposits which are harmful to the turbine engines in which they are intended to be used. Jet fuel is also used as a heat sink in turbine engines. These deposits will create maintenance problems in the turbine engines. Currently, fuel thermal stability is recognized as one of the most important properties of jet fuels. ASTM D3241 is the standard analytical procedure for rating fuel thermal stability and a fuel will either pass or fail at a given temperature. Preferred fuels for use in jet turbines will usually have a passing jet fuel thermal-oxidation tester (JFTOT) rating as measured by ASTM D3241 at 260° C.
Distillates having very high levels of saturates, such as distillates recovered from the Fischer Tropsch process, have been shown to have excellent smoke points, usually in excess of 40 mm, and low sulfur contents. As such, highly paraffinic distillates appear to be useful for blending with lower quality distillates in order to obtain a distillate blend meeting the requirements for jet fuel. What has not been recognized is that some highly paraffinic distillate components, especially those characterized by low to moderate branching of the molecule, such as those products produced by the low temperature Fischer Tropsch process, when blended with conventional distillate components can show poor thermal stability leading to the formation of unacceptable amounts of deposits.
In general, two classes of oxidation stability are of concern in this disclosure. The first is the result of low sulfur levels in the distillate, such as are found in Fischer Tropsch distillates and in fuels which have been hydrotreated to low sulfur levels. Such hydrocarbons are known to form peroxides which are undesirable because they tend to attack the fuel system elastomers, such as are found in O-rings, hoses, etc. The second source of concern is in the decline in thermal-oxidation stability as a result of the blending of the different components. For example, it has been found that highly paraffinic distillates, such as Fischer Tropsch products produced using the low temperature process, when blended with petroleum-derived distillates may result in an unstable blend which has unacceptable thermal-oxidation stability. When a blend of at least two distillate fuel components in some blending proportions result in a decline in the thermal-oxidation stability as measured by ASTM D3241, the components are described as having “antagonistic properties”.
In the case of peroxide formation, it has been suggested that the formation of peroxides in the blends may be controlled by increasing the sulfur content of the blend. See WO 00/11116 and WO 00/11117 which describe the addition of at least 1 ppm sulfur to the blend in order to prevent sulfur formation. This approach has two drawbacks. The first is that this approach does not address the problem associated with the antagonistic properties of the blending components. The second problem is that sulfur in fuels is considered an environmental hazard and it is generally desirable to reduce the level of sulfur in fuels not increase it.
The present invention is directed to a process for blending highly paraffinic distillate fuel components with low to moderate branching and conventional petroleum-derived distillate fuel components to prepare an acceptable jet fuel, wherein the two components have antagonistic properties at certain ratios which result in the a decline in the thermal-oxidation stability as measured by ASTM D3241. The invention also results in a unique product blend which is suitable for use in turbine engines.